AR-40 AR-39 DATING OF METAMORPHIC AND IGNEOUS ROCKS OF THE LISCOMB COMPLEX, MEGUMA TERRANE, SOUTHERN NOVA-SCOTIA, CANADA/

Recent geological mapping in the Meguma Terrane of southern Nova Scoti a has revealed the presence of previously unrecognized high-grade gnei sses and gabbroic rocks that form, along with previously recognized gr anitic rocks, the Liscomb Complex. Field relationships indicate emplac ement of the suites, in the sequence gneisses, gabbros, and granites, after the ca. 400 Ma Acadian deformation of the Meguma Group metasedim entary rocks. Ar-40/Ar-39 dating of amphibole and mica phases from the se suites indicates (1) an amphibole inverse isochron age of 375 +/- 3 Ma for the gneisses, and amphibole plateau ages of 371 +/- 4 and 372 +/- 5 Ma for the gabbros; (2) muscovite plateau ages of 367 +/- 3 Ma f or the gneisses, and 369 +/- 3 to 373 +/- 4 Ma for the granites; and ( 3) variably discordant biotite age spectra with integrated ages someti mes greater than the amphibole and muscovite plateau ages (371-383 Ma (n = 5) gneisses; 373-385 Ma (n = 3) gabbros; 367 and 368 Ma in granit es). The spectral discordance in the biotites is attributed to irradia tion-induced recoil in the presence of submicroscopic chlorite, wherea s the high integrated ages reflect excess argon. Collectively, the age data indicate rapid cooling of the Liscomb Complex through the argon retention temperatures of amphibole (approx. 500 degrees C) and micas (approx. 350-300 degrees C), consistent with previous Ar-40/Ar-39 dati ng of amphibole and micas in this area. This cooling history is attrib uted to rapid uplift and exhumation of the area postemplacement of the units because of its close proximity (i.e., along its northern bounda ry) to the Cobequid-Chedabucto Fault System.